I believe kennen, corki, and miss fortune are all decent counter to renekton. In the current metagame its really important to have a set of runes that include flat armor yellows in case you are against heroes like jarvan, urgot, renekton, or pantheon(although no one plays him cuz he sucks balls). then when you are laning against these champs you can buy cloth armor + 5 pots and do very well.

Teemo is probably the strongest counter to him. Like, seriously. Teemo ****s on Renekton if he's good. He can kite kite kite all day long and no matter what the croc tries he won't really be able to get in range to harass. If he tries to he will lose more hp in the damage exchange.

Kite. Just like how you deal with most melee dps, kite until they give up or die.

stimpy

^ lol

Vladimir is great versus him. You can harass him all day with your pokes and you can heal/pool to escape his aggression. If he doesn't get rage, he's pretty gimpy, and Vlad won't let him get rage.

If you are good with skillshots, I've been able to consistently hold my own against Renekton with Morgana. Lots of MPen will give her snare+pool combo surprising damage early on and if he tries to rush, her snare will allow you to escape. Granted, you're not going to be murdering him (unless he is bad), but you're also not going to be denied your own farm and you won't be giving him free reign to farm. Morg also punishes reckless players, and Renekton encourages one to play recklessly.